After a well has been drilled such as for discovery and production of petroleum products, wireline controlled apparatus is frequently employed to conduct various downhole installation, retrieval and servicing operations. Wireline equipment is utilized to install and retrieve a wide variety of downhole tools such as packers, gas lift valves, downhole safety valves, bottom hole pressure sensors and the like. Wireline equipment is also frequently utilized to run various well servicing tools such as for cleaning and treating production tubing.
At times it is desirable to establish a seal about the wireline to permit well servicing operations to be conducted within the well head and free of well pressure above the blowout preventer (BOP). More importantly, wireline BOP's are required to accommodate various emergency situations and alleviate any dangerous condition that might otherwise occur. During wireline operations the wireline BOP is typically static in an open position which allows the wireline to freely traverse the wellhead of the well without interference with any portion of the wellhead structure including the safety equipment of the wellhead.
At times during well servicing operations it will be necessary to close and seal the wireline BOP about the wireline. For example, in the event a kick develops in the open hole or in the casing of the well, the wireline BOP is closed to develop a seal to contain well pressure and prevent a blowout. With well pressure thus contained kill fluid to be pumped into the well below the seal established by the wireline BOP to shut in the well. The wireline BOP may also be closed to temporarily maintain well pressure while bleeding off the lubricator of wireline equipment to replace packoff elements. In some cases it becomes necessary to remove, add or make repairs to the riser or cable of the wireline well servicing equipment. In this case the wireline BOP is typically closed and sealed about the wireline cable to thus permit cable repairs to be made above the level of the wireline seal. Most critically, wireline BOPs are shut automatically or manually in the case of failure of the well control system above the level of the BOP.
Typically, wireline sealing elements are formed by two elastomer faced metal rams which have been contoured to fit a particular size of wireline cable. These rams are positioned in opposed relation and are actuated such that they establish sealing engagement with the wireline positioned centrally of the BOP housing. In other words, the opposed rams center the wireline within the housing and establish a seal about the wireline. The opposed rams are closed manually or hydraulically (with mechanical backup) to shut in the well. When the BOP is activated, the elastomer faces of the rams will seal around the outer surface of the cable by extruding the elastomeric material into the interstitial spaces between the armor strands of the wireline.
Ram type BOPs are designed to seal with the wireline cable in a static position. It is necessary therefore to always stop movement of the wireline before the rams are closed about the wireline to effect sealing. Short lengths of wireline cable can be stripped through the BOP as needed to repair a stranded cable but the amount of elastomeric material that can be worn away by such stripping without resulting in leakage of the BOP is limited. Pulling an appreciable length of wireline cable through the BOP will induce severe wear to the elastomeric seals of the rams and can cause damage to the rams as well, causing a more serious failure.
The highly pressurized elastomeric seal is prevented from extrusion by the close metal-to-metal fit of the ram faces and due to the fact that differential pressure moves the cylindrical rams tightly against the upper wall of the BOP housing. When the pressure of the elastomer against the surrounding surfaces of the BOP body exceed differential pressure a leak-tight seal will be effected. Wireline BOPs are designed to seal against well pressure in only one direction and therefore care must be taken to insure that they are not installed upside-down when a single set of rams is employed because the BOP will not hold well pressure. Single ram BOPs are only installed upside-down to contain pressure injected from above.
It is difficult to move the rams of wireline BOPs when the rams are closed against high differential pressures. Wireline BOPs must therefore be provided with a bypass that is used to equalize the pressure across the rams before opening of the rams is initiated. After equalization of well pressure across the rams has occurred there will be a partial relaxation of the sealing contact of the elastomeric seals with the wireline, thus reducing the extrusion of the elastomeric sealing material about the wireline and con. The effective reduction of friction makes the rams much easier to open. To allow a controlled equalization of pressure, the BOP is typically fitted with a pressure equalizing system.
Manual BOPs are operated by turning two ram operator handles on opposed sides of the apparatus to open and close the opposed rams. Manual BOP's are available in a number of sizes and ratings. Regardless of the pressure rating, they are normally used at lower pressures for standard service. Manual BOPs are typically of lighter weight and are less expensive as compared to BOPs having hydraulically energized rams. Because operating personnel must gain physical access to manual BOPs for opening and closing the rams, such personnel is typically in a more dangerous location during these activities. Also, less ram pressure can be applied with manual BOPs than with hydraulically engergized ram BOPs. In larger sizes, as commonly used for open hole work, the manual BOP offers adequate protection and is considerably lighter than a hydraulically energized BOP.
Hydraulic ram BOPs are opened and closed by hydraulic pressure acting on pistons in hydraulic cylinders. Hydraulic BOPs also have handles and stems that are used for manual backup. A hydraulic BOP can be closed manually but must be opened hydraulically. The stems must be backed out manually before the rams can be opened hydraulically.
The rams and sealing elements of wireline BOPs have grooves that are sized for the wireline cable diameter being employed. As the rams are closed, the cable is guided by the rams or other cable guide elements into the grooves of the sealing elements. BOPs are provided with "integral guide" rams that prevent cable damage as the cable is guided and centralized during ram closure. If the rams of hydraulic wireline BOPs are to be left closed for a long period of time or in case of hydraulic failure, the manual screw jack can be used to hold the rams in the closed position. To open a hydraulic wireline BOP, the mechanical backup must be in the open position before shifting the "selector" to its open position and hydraulically pumping the rams to their open positions.
Multiple ram BOPs, typically dual wireline BOPs, are utilized to provide a backup in case of failure of the primary set of sealing rams. More importantly is the fact that gas will migrate through the interstices between the inner and outer armor in wireline cables. In the event that the lubricator of the wireline equipment would need to be removed for some reason, the leaking gas would quickly present a significant problem from the standpoint of danger to personnel. To alleviate this problem, a second (tandem) BOP is added. This BOP is inverted (because wireline BOPs hold pressure in only one direction) and a port is added between the two BOPs. High pressure grease (above well pressure) is injected into the flow passage between the upper and lower sets of rams. Under high pressure, the grease is caused to migrate into the interstitial space between the inner and outer armor of the wireline and thereby effects a grease seal to prevent gasses from escaping. In some cases a triple BOP is also available which provides a backup in the case of primary ram failure. A triple BOP also provides a method for injection of grease between the BOPs if needed. Multiple ram BOPs are available in a single forged body for lighter weight and more compact size.
As explained, it is frequently necessary to employ dual and sometimes triple wireline BOPs and to provide for grease injection between them in order to effect a proper wireline seal and prevent migration of gasses through the interstices of the wireline. Since the flow passage between the stacked or multiple wireline BOPs is typically of significant length and is at least as great as the diameter of the flow passage through the wireline, a considerable volume of injected grease is necessary to accomplish efficient sealing. Also, injection of a sufficient volume of grease to fill the flow passage and develop a hydraulic seal with the wireline can require a considerable period of time. It is desirable to minimize the time required to develop an adequate hydraulic seal with the wireline to thus promote the safety of the sealing operation. Obviously, employment of dual and triple ram BOPs whether of the manual or hydraulic type can result in considerable expense Moreover, the stacking of dual and triple wireline BOPs ordinarily results in a BOP riser of significant height. In many cases there is little room to accommodate multiple wireline BOPs of this nature. Accordingly, it is desirable to provide a single ram wireline BOP having the capability of providing both elastomeric sealing by means of opposed rams and to provide hydraulic sealing by means of grease injection.
It is therefore a primary feature of the present invention to provide a novel single ram wireline BOP, whether energized manually or hydraulically, which achieves an efficient mechanical seal about braided wireline to efficiently contain well pressure below the rams.
It is also a feature of this invention to provide a novel single ram wireline BOP that has the capability of grease injection about the wireline to thus provide an additional hydraulic seal about the wireline cable to prevent the flow of gasses through the interstices formed by the armor of the cable.
It is an even further feature of this invention to provide a novel single ram wireline BOP incorporating both mechanical and hydraulic sealing and wherein hydraulic sealing is effected quickly by means of a minimal volume of injected grease.